The present invention relates to a chemimechanical process for removing silica from nonwood plant fibre sources.
There is growing interest in using nonwood plant fiber sources, such as wheat straw, flax and hemp, for pulping and papermaking. These and other nonwood lignocellulosic materials can find value-added utilization that would enhance the profitability of farm production.
As wood fiber shortages are predicted in the future, nonwood plants are believed to be a sustainable fiber source to potentially supplement the use of wood fibers in paper applications. Market forces and legal requirements may stimulate the production of paper that contains nonwood plant fibers, as exemplified by experience with recycled fibers.
The art of papermaking was originally developed using nonwood plant sources, whereas the production of pulp and paper from wood is a relatively recent development. Pulping processes can be broadly divided into two large categories: chemical pulping and mechanical pulping. Chemical pulping involves using chemical reactions to solubilize lignin and produce individual fibers or pulp from lignocellulosic raw materials. Within the mechanical pulping category, there are many processes that involve varying combinations of chemical, mechanical and thermal treatments to effect fiber separation, remove some lignin and other chemical components from the original fibers, or increase the brightness or papermaking strength of the resulting fibers. Chemimechanical pulps (CMP) from wood are produced by processes in which the raw material is treated with weak solutions of pulping chemicals such as sulfur dioxide, sodium sulfite, sodium bisulfite or sodium hydrosulfite, followed by mechanical defibration.
One of the problems associated with the chemical pulping of nonwood plants is the difficulty in recovering the cooking chemicals from the spent cooking liquor (“black liquor”), which is a result of the relatively high levels of silica found in most nonwood plant fibers, as compared to wood. During alkaline cooking of nonwood plant fibers, this silica is dissolved and is subsequently removed from the fibers via the black liquor stream, which is sent to the chemical recovery system for conversion into fresh cooking liquor. The silica-laden liquor causes scaling and fouling in evaporators, concentrators and the recovery boilers, resulting in inefficient operation and increased downtime for clean-outs. The inability to recover cooking chemicals from silica-laden black liquor results in increased operating cost and effluent treatment system loading.
Alternatively, mechanical pulping seems to be more suitable for raw materials with higher silica content, particularly wheat and rice straws, since the silica is not dissolved to the same extent as for chemical pulps and will for the most part remain with the fibers throughout the pulping and bleaching process. Mechanical pulping also generates a minimal volume of effluent, thus reducing the environmental impact. However, mechanical pulping generally results in pulp of lower quality. Significant amounts of lignin are left with the mechanical pulp, making it weaker and more difficult to bleach to high brightness than its chemical pulp counterpart.
In U.S. Pat. No. 6,183,598, a process for recovering alkali and heat energy from black liquor is disclosed. The black liquor is the result of a chemical process where the nonwood plant material is heated with an alkaline cooking liquor containing sodium hydroxide. The black liquor contains a significant amount of silicate ions. Recovery of the sodium hydroxide using lime is usually impeded by the formation of calcium silicate, which makes recycling of the lime difficult or impossible. The solution proposed in this patent is to treat the black liquor with carbon dioxide to precipitate silica and lignin. The solids are then removed and the remaining black liquor is evaporated and burnt to generate heat and a sodium carbonate melt, from which carbon dioxide is formed. The carbon dioxide is then reused to treat the black liquor. However, with the precipitation of lignin, there may also be a loss of some inorganics that will limit the potential recovery efficiency. Also, this process is energy intensive because of the heat lost by precipitation of some lignin, which would otherwise be burnt in the recovery boiler to generate steam.
There is still a need in the art for processes which may permit pulping of nonwood plant fibers while allowing chemical recovery despite the high silica content of the fibers.